The field of the present invention is fish canning processes.
The canning of tuna and tuna-like fish, including albacore, bluefin, bonito, skipjack, bigeye, tonggol, euthynnus and yellowfin, normally includes precooking. This precooking is to allow easy removal of the skin and the skeleton. The precooking step typically employs hot water from 180.degree. F. to 200.degree. F. or saturated steam with temperatures ranging up to 214.degree. F. One such process and the equipment associated therewith is described in U.S. Pat. No. 3,709,142, the disclosure of which is incorporated herein by reference. The precooking process is necessary but can have certain undesirable results. It reduces the weight of the flesh and can adversely impact the quality of the final product.
In most operations, the fish are delivered in a thawed state with a core temperature of about 30.degree. F. The practice is to raise the backbone temperature of the fish during a precooking process to a range of 120.degree. F. to 160.degree. F. depending on the size and species. Following the precooking, the fish may be cooled by air cooling, water streams, water misting or other conventional processes. Once cooled, the skin and skeleton are typically removed by hand. The flesh, devoid of bones and skin, is then placed in cans and fully cooked in a pressure vessel, also known as a steam retort.
In precooking the fish, mobile racks or trays are typically pushed into a large chamber. The doors are closed and generated steam is introduced. The steam is typically saturated when introduced and, therefore, is supplied at approximately the boiling point of water. Some overpressure often occurs because of the location and size of the vents which may typically be liquid drains. As such precookers are not pressure vessels, it is important to avoid more than a very small pressure above atmosphere.